This invention relates to structures for conversion of energy, and more particularly to the conversion of radiant energy into electricity by photovoltaic panels which are suitable for relatively high current and low voltage applications.
Photovoltaic panels are formed by a photo-sensitive element that is sandwiched between two conductors. A typical photo-sensitive element is a semiconductor such as silicon or germanium either in crystalline or amorphous form. To allow radiant energy access to the semiconductor, at least one of the contacting conductors is transparent to the energy being converted. In addition, where the semiconductor is formed by deposition, a transparent support, i.e., substrate such as glass, is the base upon which the deposits are made. As a result the semiconductor energy converter has a transparent conductor on a glass support with an overlying semiconductor, followed by a further conductor which need not be transparent.
Since transparent conductors commonly present a finite sheet resistance, on the order of 5 to 15 ohms per square for materials such as doped tin oxide, it is necessary to limit the travel path in the transparent conductor for collected current generated by the semiconductor to avoid significant resistance losses. As a result, it is common practice to periodically scribe the transparent conductor, giving it a maximum width on the order of about 1 centimeter in order to avoid excessive resistance losses. The resulting structure then has approximately 30 cells per foot, with each width between scribes constituting a cell. The cells are then connected in series.
In order to make the desired series connection, it is necessary to make further scribings on successive overlying layers. Besides the first scribings which divide the conductive transparent oxide into isolated strips on the glass substrate, it is also necessary to scribe the overlying film or layer of semiconductor, such as amorphous silicon. When the rear electrode (conductor) is deposited upon the semiconductor, it is necessary to scribe it as well.
Consequently, in the fabrication of the typical photovoltaic panel, it necessary to make three successive sets of scribings for three successive layers. These scribings must be accurately registered and must be as close as possible in order to limit the extent of photovoltaically inactive or dead area. This is the area that is encompassed by the scribe boundaries between which the photovoltaic panel is inactive. It is apparent that the wider the area that is subjected to scribing, the greater is the amount of dead area in the panel and consequently the greater is the reduction in energy conversion. On the other hand, when an attempt is made to move the scribings too closely together, there is a danger of the scribings overlapping or intersecting, resulting in short circuiting.
Another characteristic associated with conventional photovoltaic panels formed by series-connected cells is the high open circuit voltage of the panel. Each cell has a voltage level, in the case of amorphous silicon, on the order of 0.8 volt. Where there are 30 cells per foot, the total open circuit voltage of the series connected cells is on the order of 24 volts.
While a 24 volt output is suitable for many applications, it is not usable where low voltages and high currents are required. For example, in the electrolysis of water to produce hydrogen for fuel cells, it is desirable to have a relatively low voltage and high current. It is not possible, with current technology, to realize low voltage photovoltaic panels that are efficient and suitable.
Accordingly, it is an object of the invention to reduce the amount of dead space associated with the scribing of successive layers in photovoltaic panels. A related object is to achieve a larger active area for a photovoltaic panel of a given panel size.
Another object of the invention is to reduce the number of scribing steps needed in the formation of a photovoltaic panel. A related object is to simplify the scribing of the layers used in forming photovoltaic panels.
Still another object is to reduce the criticality associated with the registration of successive scribing steps. A related object is to completely eliminate the need for the registration of successive scribes.
Still another object of the invention is to achieve a large area panel that can be used for low voltage and high current applications while simultaneously having a small percentage of inactive area.